おはよう The combination of "o" and "u" is usually pronounced as long "o". In Japanese, this can result from various character combinations (yo/よ + u/う in this example). — Tae Kim's Japanese Grammar flashcard

こんにちは

Note that the Japanese "ha"-character (は) is pronounced "wa" in this case.

こんばんは

Again note that the Japanese "ha"-character (は) is pronounced "wa" in this case.

ありがとう

Note the combination of "o" and "u", which is pronounced as a long "o". It's a result of to/と + u/う in this example.

ありがとう ございます

おはよう ございます

すみません